1. Field of the Invention
The present invention relates to routing connections in a telecommunications network, and, more particularly, to capacity allocation for paths through nodes of the network for path restoration.
2. Description of the Related Art
In interconnected optical communications networks, a user establishes a connection between a source node and a destination node with a stream of data that is transferred through the network over a network path. Optical networks are typically characterized by a set of micro-mechanical optical switches (nodes) connected via optical links. A network path for a connection between a given source-destination (node) pair is defined by a set of nodes (the source and destination node pair and any intermediate nodes) interconnected by a set of links coupled to the nodes carrying the data stream, or flow, of the connection.
Service restoration is an important requirement of optical networks. If a network element fails, such as a node (optical switch) or link (optical fiber), the failure causes one or more particular wavelength paths to fail, and affected traffic flow(s) must be restored using an alternate path within a very short interval (e.g., 50 ms). To accomplish relatively rapid restoration times, provisioning identifies, for each wavelength path, two paths through the network: a primary (active) path and a secondary (backup) path. The backup path is link disjoint (active and backup paths do not share links) or node disjoint (active and backup paths do not share either nodes or links) with the primary path. The capacity of links in the backup path assigned to a corresponding primary path (e.g., wavelength), or, for network bandwidth usage efficiency, the capacity may be shared between links of backup paths for different primary paths, depending on the type of restoration desired. Optical network capacity design typically accounts for restoration needs to route disjoint secondary paths with possible sharing.
A connection may be considered a fast path restorable (FPR) connection if the backup path switching configurations are fixed at the time of setting-up the connection in the active path and no reconfiguration is required after a failure in the active path. FPR connections are of two types: FPR connections with no shared backup (NSB FPR connections) and FPR connections with shared backup (SB FPR connections).
NSB FPR connections have two link disjoint paths reserved for the connection, where no capacity of links in the backup path is shared with other backup paths. Two methods for restoration may be employed for NSB FPR connections. In the first method, one disjoint path is the active path with all traffic switched to the other disjoint path upon an active path failure. In the second method, the source transmits the same traffic on both disjoint paths, and the destination picks the path from which to receive the traffic based on some metric, such as signal-to-noise ratio (SNR) or bit error rate, for the signal received from each path. Both methods of restoration for NSB FPR connections, however, exhibit relatively poor bandwidth usage efficiency from the overall network capacity standpoint.
SB FPR connections, while exhibiting increased bandwidth usage efficiency, are subject to certain constraints to be feasible. First, active paths between the same source-destination pairs may share backup path bandwidth (i.e., may share capacity of backup path links). Second, if two active paths share backup paths, then the backup paths are shared end-to-end. To illustrate aspects of sharing in FPR connections, FIG. 1 shows a network 100 of nodes N1-N5 connected by links lij, i,j integers and 1≦i,j≦5. As shown in FIG. 1, each link lij has a capacity of 100 units of bandwidth. A connection between source s(1) and destination t(1) may be established with a desired demand (bandwidth capacity requirement) of 200 units. If no restoration is required, the network may route up to 300 units of demand in the active path. The active path is defined as 100 units over the path defined by N1-N2-N5, 100 units over the path defined by units N1-N3-N5, and 100 units defined over the path defined by N1-N4-N5.
If the connection is established as a NSB FPR connection, the maximum amount of capacity (packet or data flow) that may be accommodated by network 100 is 150 units. FIG. 2 illustrates the network of FIG. 1 with an exemplary NSB FPR connection accommodating 150 units of demand with no capacity sharing in the backup path. In the figures, the dashed line indicates the backup path and the solid line indicates the active path. As shown in FIG. 2, for example, 75 units of demand is routed through a first path defined by N1-N3-N5, and 75 units of demand is routed through a second path defined by N1-N4-N5. The backup path for the first path is a) 50 units of demand reserved on the path defined by N1-N2-N3 and b) 25 units of demand reserved on the path N1-N4-N5. Similarly, the backup path for the second path is a) the remaining 50 units of demand reserved on the path defined by N1-N2-N3 and b) 25 units of demand reserved on the path N1-N3-N5.
However, if the connection is established as a SB FPR connection, network 100 may accommodate 200 units of demand. For example, the active path may be defined as 100 units over the path defined by N1-N3-N5 and 100 units over the path defined by units N1-N4-N5. The backup path is defined as 100 units over the path defined by N1-N2-N5. Thus, the 100 units of capacity of the path N1-N2-N5 is completely shared by the paths N1-N3-N5 and N1-N4-N5. Such SB FPR connection is shown in FIG. 3.